BIOTROPIA Vol. 28 No. 2, 2021: 156 - 164 DOI: 10.11598/btb.2021.28.2.1280 156 SULFATE AMMONIUM FERTILIZER ON THE OFF-SEASON PRODUCTION OF SNAKE FRUIT (Salacca sumatrana Becc.) RASMITA ADELINA1*, IRFAN SULIANSYAH2, AUZAR SYARIF2, AND WARNITA2 1Agrotechnology Study Program, Faculty of Agriculture, Universitas Graha Nusantara, Padangsidimpuan 22715, Indonesia 2Agriculture Science Study Program, Faculty of Agriculture, Universitas Andalas, Padang 25136, Indonesia Received 5 July 2019/Accepted 9 January 2020 ABSTRACT Salacca sumatrana (Becc.), known locally as the Sidimpuan snake fruit, is one of the specialties prime local commodities of Padangsidimpuan City in Sumatra. The fruit is known for its sweet, sour and astringent taste which differentiates it from Pondoh and Balinese snake fruits. Recently, the snake fruit farmers have noticed a continuous decrease in production resulting from the failure in its fruit-setting, particularly during the off-season. The use of fertilization and drip irrigation in the off-season had been currently explored as part of the solution. Hence, this research investigates the use of these methods in overcoming the fruit setting failure and guaranteeing subsequent production of Sidimpuan snake fruit all-year round. Specifically, this study aimed to determine the optimal dosage of ammonium sulfate fertilizer and drip irrigation for fruit setting during the off- season. This research used a split-plot design with the main plot for drip irrigation and the subplot for ammonium sulfate. The observed parameters included the number of flower and fruit bunches, fruit set percentage and a nutrient analysis of the leaves. Drip irrigation significantly affected the fruit setting percentage and the number of harvested fruit bunches. The best treatment combination was at 400 g ammonium sulfate fertilizer per plant and drip irrigation of 3,000 mL/plant. The fertilization period in July-September produced an off season harvest that was comparable to the fruit set percentage (10.76% difference) and number of fruit bunches (25.65% difference) that were observed in the April-June fertilization for the on-season harvest. This indicated that applying ammonium sulfate with drip irrigation could overcome fruit set failure in Sidimpuan snake fruit, particularly, during the off-season. Keywords: drip irrigation, off-season, production, snake fruit, sulfate ammonium INTRODUCTION Sidimpuan snake fruit (Salacca sumatrana Becc.) is one of the specialty products locally produced primary commodities of Padang, Sidimpuan City. The fruit is known for its uniquely sweet, astringent and sour taste, differing from the Pondoh and Balinese snake fruits and other types of snake fruits. The species is spread throughout the sub-districts in the southern part of the Tapanuli Regency, predominantly at the Districts of Angkola Barat, East Angkola, South Angkola, and Marancar. Sidimpuan snake fruit production shows high development potential in South Tapanuli covering approximately 19,155 ha with a production potential of up to 30 tons/ha (BPS 2015). Hence, the development of an optimal cultivation technology is necessary. Currently, the crop production undergoes high fluctuations between the main harvest season (on season) and harvest outside this season (off season) resulting in a continuously declining production. This decreasing production is due to the fruit-set failure in the off season which is caused by a number of adverse environmental factors, particularly, those which do not support the production process, including low rainfall and few rainy days, and low soil nutrient content resulting in the lack of vital nutrients as indicated by low nitrogen, phosphorus and potassium content in the leaves (Rai et al. (2010). *Corresponding author, email: rasmita301271@gmail.com Sulfate ammonium fertilizer on the off-season production of snake fruit (Salacca sumatrana Becc.) – Adelina et al. 157 For the Sidimpuan snake fruit to bear well outside the main harvest season, off-season treatments need to be applied. These would include nutrients supplementation through fertilizer application and simple drip irrigation technique. Fertilization with ammonium sulfate and potassium chloride and simple drip irrigation were tried to meet the nutritional and water requirements of Sidimpuan snake fruit in increasing its growth, fruit formation and production, out of season. Under drip irrigation, fruit-setting was at 75.30% while that of no drip irrigation it was at 59.94% (Rai et al. (2010). Ammonium sulfate fertilizer provides nitrogen and sulfur and potassium chloride fertilizer as the main source of potassium which plays a role in plant growth and development. The availability of nitrogen and sulfur nutrients by fertilizing with ammonium sulfate at a dosage of 300 g had increased Suwaru snake fruit production (Sudaryono 2005) while the use of simple drip irrigation had efficiently and optimally meet the plants’ water requirements. Hence, this study was aimed to obtain the best dosage combination of ammonium sulfate fertilizer and simple drip irrigation techniques to optimize fruit setting and to increase the Sidimpuan snake fruit production in the off- season thereby optimally producing fruit throughout the year. MATERIALS AND METHODS Time and Location of Research The present study was conducted in April 2018 until September 2018 at the Palopat Maria Village Padangsidimpuan Hutaimbaru Subdistrict, Padangsidimpuan City (between 010 28’19’’ - 010 18’ 07’’ N and 990 18’ 53’’ - 990 20’ 35’’ E). Research Methodology Sixty (60) trees from among the 20 to 30- year-old productive snake fruit trees were measured in the study applying the split plot design, consisting of three replications. The main plot is the application technique either, with drip irrigation of 3,000 mL/plant/day (P1) or with no irrigation (P0). The simple drip irrigation installation included infusion bottles, infusion tubes, drippers, plastic hoses and water pumps. The subplots for the fertilization technique consisted of plots without fertilization (P0) and plots with ammonium sulfate fertilizer applied at 250 g/plant + 40 g potassium chloride/plant (P1), 300 g/plant + 40 g potassium chloride/plant (P2), 350 g/plant + 40 g potassium chloride/plant (P3), and, 400 g/plant + 40 g potassium chloride/plant (P4). Hence, the study consisted of 10 treatment combinations with 3 replications and 2 plants per plot, totalling to 60 plants. Fertilizer applications were carried out in 2 periods; fertilization for the April - June 2018 period was carried out on 14 March 2018, while fertilization for the July - September 2018 period was carried out on 22 July 2018. Previously, fertilization had been done in August 2017 and December 2017. Fertilization was carried out by immersing the appropriate amounts of ammonium sulfate and potassium chloride fertilizer according to treatment into a 10 - 15 cm deep fertilizer groove in the soil that is 50 - 60 cm from the base of the stem. Watering was a simple irrigation system that involved water movement carried out by gravity. Drip irrigation equipment included a plastic tube as a water storage container, an infusion hose installation equipped with a dripper at the end that released water at a rate of 250 mL/30 minutes with a watering volume of 3000 mL/day. The drip irrigation was given daily for 6 hours from 10 am - 4 pm. However, if it rains very heavily, the drip irrigation was not applied. The observed parameters were the fruit-set percentage, number of flower and fruit bunches, number of harvested fruit bunches, relative water content (RWC) and the Nitrogen, Phosphorus and Potassium content on the leaves. The number of flower bunches and fruits were counted once every two weeks on each sample plant. Analysis of leaf nitrogen, phosphorus and potassium content was carried out once every fertilization period. Three leaves taken from each sample plant for laboratory analysis were cleaned, ovendried at 70 oC, then blended and sieved using a 0.5 mm grid sieve. The total nitrogen was determined using the Kjeldahl semi-micro method, while the dry ashing method was used for determining phosphorus and potassium content. Phosphorus concentration was measured with a UV-VIS spectrophotometer, while potassium BIOTROPIA Vol. 28 No. 2, 2021 158 concentration was determined by using a flame photometer. Data were analyzed using ANOVA and if differences among treatments were significant, Duncan's multiple range test was applied. RESULTS AND DICUSSION Fruit Set Percentage (%) During the April-June fertilization period (on season), the highest fruit set percentage was at 76.949% indicating a significant increased caused by using the drip irrigation (Table 1). The highest fruit set percentage of 75.615% also occurred at the 300 g/plant ZA dosage of ammonium sulfate fertilizer. The lowest response was from no irrigation (54.478%) nor fertilization (47.115%). Drip irrigation has increased the relative water content of the leaves. Hence, the high fruit-set percentage with drip irrigation was a result of the high relative water content (RWC) of leaves which in turn impacted the chlorophyll and potassium content (Sunarka 2015). Watering the soil then improves its chemical properties encouraging root growth and increasing physiological activity in the snake fruit plants, thereby improving their fruit- setting capacity. In the July-September fertilization period (off season), the fruit set percentage (58.863%) was not significantly affected by drip irrigation treatment (68.673%). This was during the wet season, so the additional water was not necessary. The number of bunches formed and flower fall is influenced by environmental factors (Adijaya et al. 2013). In the dry months, the flower fall usually increases thereby reducing the number of bunches formed. The use of ammonium sulfate fertilizer also had a statistically significant impact on the fruit setting with the highest response at 400 g per plant (77.792%). In another study, drip irrigation had significanly affected the out of season fruit production in dry conditions showing a fruit set percentage of 75.30% (off-season) and 93.13 % (on-season), compared to 59.94% and 61.67%, respectively, without irrigation (Rai et al. 2014). In the July-September fertilization period of this study, the drip irrigation resulted in a significantly higher fruit set percentage. Analysis of variance indicated that the effect of the combination of drip irrigation treatment and ammonium sulfate fertilizer dosage on the percentage of fruit formation was not significant (Table 2). However, the treatment combination that resulted in the best fruit set percentage in the April-June fertilization period was at a dosage of 350 g fertilizer per plant with drip irrigation (91.667%) (Table 2). Whereas, in the fertilization period of July-September, the highest percentage of fruit set was with 400 g with drip irrigation (83.997%) (Table 2). In another study, the combination of drip irrigation and nitrogen fertilizer had also significantly increased the yield of cotton plants (Gossypium hirsutum) (Zhong & Bai 2013). Moreover, under medium irrigation, the ratio of dry matter in nutritional organs to reproductive organs was also increased (Zhong & Bai 2013). Table 1 The average percentage of fruit set, number of flower bunches, fruit and number of harvested fruit bunches with drip irrigation treatment and different dosages of ammonium sulfate fertilizer applied in April - June and July - September 2018 Treatment Fruit Set (%) Number of bunches Number of harvested fruit bunches Flower Fruit Apr-Jun Jul - Sept Apr-Jun Jul- Sept Apr-Jun Jul- Sept Apr-Jun Jul- Sept Without irrigation 54.478a 58.863 19.267b 15.000 10.533 8.467 9.933 12.467a Drip irrigation 76.949b 68.673 16.467a 14.267 12.733 9.467 12.067 18.200b Without fertilizer 47.115a 52.620a 16.333 16.333 7.667a 7.833a 10.500 13.000 ZA 250 g 59.708ab 57.800ab 17.833 13.500 0.500ab 7.667a 9.000 14.667 ZA 300 g 75.615b 68.620ab 18.667 14.833 14.167b 10.000ab 11.000 13.833 ZA 350 g 74.443b 62.008ab 18.333 14.167 12.833b 8.167ab 10.167 18.000 ZA 400 g 71.687b 77.792b 18.167 14.333 13.000b 11.167b 14.333 17.167 Note: Means with different superscripts within a column are significantly different at P≤0.05, with comparisons performed using DMRT. Sulfate ammonium fertilizer on the off-season production of snake fruit (Salacca sumatrana Becc.) – Adelina et al. 159 The application of ammonium sulfate fertilization and drip irrigation had increased the percentage of fruit set in both fertilization periods. This indicated that these production techniques will be able to minimize the production fluctuations in snake fruit between the harvest season (on season) and small harvests (off season). Other study results also showed that both irrigation and nitrogen fertilization promoted cotton growth and yield (Zhuan et al. 2017). Drip irrigation technique had also increased the fruit set percentage of Gula Pasir snake fruit during the off season in a dryland ( Rai et al. 2014). Number of Flower Bunches During the April - June (off season) fertilization period, the flower formation was significantly affected by the drip irrigation as a single factor (Table 1). No irrigation treatment resulted in the most numerous flower bunches formed (19.267). Ammonium sulfate fertilizer dosage at 300 g/plant also produced the highest average number of flower bunches (18.667) (Table 1). During the July - September (on season) fertilization period, the drip irrigation nor the fertilization did not significantly influence the flower formation (Table 1). However, there was a tendency for more flower bunches with the no-irrigation treatment. This absence of significant difference with the irrigation technique is similar to that found by Jose et al. (2013) who used regulated deficit irrigation techniques to improve fruit size. Reduced irrigation and fruit thinning were found to affect carbon allocation within the tree by altering a number of interrelated factors (photosynthesis, location and number of competing sinks, storage capacity, and transport) that control the carbon partitioning in fruit trees. Other factors affecting flowering could be interspecific differences and different crop seasons which was probably caused by variable atmospheric conditions during vegetation periods (Greiner & Kohl 2014). No significant relationship exists between fertilizer dosage and flower formation. In a study on sweet orange plants, the application of certain dosages phosphorus and potassium compound fertilizer had only a small effect on the number of flowers formed (Ramadhan et al. 2015). Other factors that cause unfertilized plants to flower as prolifically as those treated with fertilizer could be the irrigation and less than optimal water management. Table 2 Average fruit set percentage (%) with the combined drip irrigation and ammonium sulphate fertilization in April-June 2018 and July-September 2018 Treatment Dosage of ammonium sulfate fertilizer (gram) Average 0 250 300 350 400 I* II** I II I II I II I II I II Without irrigation 34.140 44.330 50.437 52.503 67.603 65.157 57.220 60.740 62.990 71.587 54.478 58.863 Drip irrigation 60.090 60.910 68.980 63.097 83.627 72.083 91.667 63.277 80.383 83.997 76.949 68.673 Average 47.115 52.620 59.708 57.800 75.615 68.620 74.443 62.008 71.687 77.792 Notes: * = fertilization in April–June 2018; ** = fertilization in July-September 2018 Table 3 Average number of flower bunches with ammonium sulfate fertilizer treatment Treatment Dosage of ammonium sulfate fertilizer (gram) Average 0 250 300 350 400 I* II** I II I II I II I II I II Without irrigation 16.667 19.333 19.667 12.000 18.667 16.333 21.667 14.333 19.667 13.000 19.267 15.000 Drip irrigation 16.000 13.333 16.000 15.000 18.667 13.333 15.000 14.000 16.667 15.667 16.467 14.267 Average 16.333 16.333 17.833 13.500 18.667 14.833 18.333 14.167 18.167 14.333 Notes: * = ammonium sulfate fertilization in April-June 2018; ** = ammonium sulfate fertilization in July-September 2018. BIOTROPIA Vol. 28 No. 2, 2021 160 The combination of drip irrigation and the different ammonium sulfate dosages did not significantly affect the number of flower bunches. Similar results were observed when new flower buds formed spontaneously every 1 - 1.5 months at the base of the leaf midrib but were uninfluenced by the combined irrigation and fertilization treatments (Adelina 2017). Furthermore, it was then assumed that the two treatments would influence the formation of new flower buds, later, when the fruit bunches were formed and until finally, when the fruit bunches were harvestable (Table 3). The application of certain dosages of compound fertilizer phosphorus and potassium had also a minimal effect on the number of flowers in sweet orange plants (Ramadhan et al. 2015) Number of Fruit Bunches The number of fruit bunches in the April- June fertilization period was not significantly affected by the drip irrigation in which the irrigated plants produced an average of 12.733 bunches per plant. In contrast, drip irrigation had a significant effect on both growth traits and fruit production indicating that it was important in zones with water limitations (Loewe & Delard 2016). The different dosages of ammonium sulfate fertilizer has significantly affected fruit bunch formation but the best dosage appeared to be at 400 g with 13.000 bunches of fruit per plant, and the 250 g at only 10.500 bunches (Table 1). In the July-September fertilization period, the drip irrigation had no significant effect on the number of fruit bunches formed but it appeared to give a slightly higher number of fruit bunches (9.467) compared to the no-irrigation technique (8.467) (Table 1). During this period, there was sufficient rain so the effect of irrigation in the formation of fruits seemed neligible. The drip irrigation was more effective during the dry season rather than on the rainy season (Biswas et al. 2016). The same effect happened in the increased yield of tomato associated with the increasing amount of irrigation water (Biswas et al. 2016). The optimal dosage of ammonium sulfate fertilization for fruit bunch formation was at 400 g with 11.167 bunches and the 250 g only yielding 7.667 bunches, showing a statistically significant difference (Table 1). The use of nitrogen fertilizer and optimal water application were also recommended for the summer production of cotton (Zhuan et al. 2017). In the April-June fertilization period, the combined treatments that resulted in the formation of the largest number of fruit bunches was drip irrigation with the 300 g fertilizer that produced 15.667 bunches (Table 4). This interaction between irrigation and fertilization influenced most of the plant physiological functions and growth (Wang et al. 2018). The same results indicating better economic production were observed in the use of fertilizer with daily drip irrigation for corn growing on a semi-arid area (Chauhdary et al. 2017). Number of Harvested Fruit Bunches In the April-June fertilization period, neither drip irrigation nor dosages of ammonium sulfate fertilizer significantly affected the number of harvested fruit bunches. However, drip irrigation with 12.067 bunches and 400 g of fertilizer with 14.333 bunches appeared to give the best fruit harvest compared to 9.000 bunches from the 250 g of fertilized plants (Table 1). Ammonium sulfate fertilizer, as the main source of nitrogen, is crucial in increasing the crop production process. Nitrogen application has increased the seed yield of coriander (Coriandrum sativum L.) in a linear manner and the application of 60 kg/ha has improved the yield by 40% (Alil et al. (2015). Table 4 Average number of fruit bunches with combined drip irrigation and ammonium sulfate fertilizer treatment Treatment Dosage of ammonium sulfate fertilizer (gram) Average 0 250 300 350 400 I* II** I II I II I II I II I II Without irrigation 5.667 7.667 10.000 6.333 12.667 11.000 12.000 8.000 12.333 9.333 10.533 8.467 Drip irrigation 9.667 8.000 11.000 9.000 15.667 9.000 13.667 8.333 13.667 13.000 12.733 9.467 Average 7.667 7.833 10.500 7.667 14.167 10.000 12.833 8.167 13.000 11.167 Notes: * = combined treatment in April-June 2018; ** = combined treatment in July-September 2018. Sulfate ammonium fertilizer on the off-season production of snake fruit (Salacca sumatrana Becc.) – Adelina et al. 161 In the July-September 2018 fertilization period, drip irrigation had resulted in a statistically significant 18.200 harvested fruit bunches while any dosage of the ammonium sulfate fertilizer did not. The plants applied with the drip irrigation also produced a higher number of harvested bunches compared to those plants that were not given (Rai et al. 2013). The best yield was at the 350 g fertilizer dosage with 8.000 bunches of harvested fruit compared to 12.467 bunches from the non-irrigated unfertilized plants (Table 1). The non-significant effect observed on the combined irrigation and ammonium sulfate fertilizer treatment on the number of harvested fruit bunches implied that no real interactions existed between the two treatments. However, 22.667 fruit bunches resulted from a combination of 400 g fertilizer with drip irrigation and 7.000 bunches from the 250 g with no-drip irrigation (Table 5). The lack of influence of ammonium sulfate fertilization suggested that drip irrigation alone can meet certain needs of Sidimpuan snake fruit plants. The use of organic fertilization has also increased the fruit set percentage (ability to bear fruit) of snake plants (Dewi 2014). The failure of fruit development from the flowers of the Gula Pasir snake fruit was more likely due to other environmental and plant physiological factors rather than the application of fertilization (Rai et al. 2010). The development pattern of snake fruit plant production and distribution is strongly influenced by physiographic environments such as the altitude, land, rainfall, and air temperature (Cahyani et al. 2013). These environmental factors include particularly, low rainfall and number of rainy days which lower the relative water content (RWC) of leaves thereby disrupting the metabolic processes. The average RWC of leaves in the July-Sept 2018 period was higher than those of leaves in the April-June period (Fig. 1). The positive impact resulted in a higher average number of fruit bunches harvested in the July-September period using the drip irrigation. This application of irrigation also resulted in greater fresh biomass, fresh leaf yield, and dry leaf yield in stevia plants (Benhmimou et al. 2018). The higher leaf RWC with the drip irrigation treatment showed that irrigation increased the water content of the plant tissues thereby positively affecting the physiological processes as indicated by the increased plant ability to take up nutrients (Rai et al. 2014). The nitrogen, phosphorus, potassium content and RWC of leaves in the July- September fertilization period was higher than those of the April-June 2018. Very high differences were found in the nitrogen content and RWC of leaves (Fig. 1). The leaf nitrogen content in the July-September fertilization period was higher (2.194%) than in the April- June fertilization period (1.384%). Nitrogen content and the RWC varied in a similar way to the percentage of fruit set and the highest number of fruit bunches obtained in the July- September fertilization period compared to those of the April-June 2018. The low productivity of Gula Pasir snake fruit was influenced by the low level of nitrogen in the leaves (Rai et al. 2010; Dewi 2014). This nutrient deficiency has influenced the plant physiological processes resulting in the failure of flower development into fruit due to photosynthate deficiency indicated by sucrose content, total sugar, and reducing sugars in the Table 5 Average number of harvested fruit bunches with the combined drip irrigation and ammonium sulfate fertilizer treatments in April-June 2018 and in July-September 2018 Treatment Dosage of ammonium sulfate fertilizer (gram) Average 0 250 300 350 400 I* II** I II I II I II I II I II Without irrigation 10.000 11.667 7.000 12.000 9.000 11.000 8.333 16.000 15.333 11.667 9.933 12.467 Drip irrigation 11.000 14.333 11.000 17.333 13.000 16.667 12.000 20.000 13.333 22.667 12.067 18.200 Average 10.500 13.000 9.000 14.667 11.000 3.833 10.167 18.000 14.333 17.167 Notes: * = combined treaments in April-June 2018; ** = combined treatments in July-September 2018. BIOTROPIA Vol. 28 No. 2, 2021 162 Figure 1 Average nitrogen, phosphorus, potassium and relative water contents of leaves leaves at low interest due to high competition in fighting photosynthesis. Fruit weight and fruit yield have been found to be significantly and positively correlated with nitrogen, phosphorus, potassium, iron, zinc and copper contents of the citrus var. Kinnow mandarin leaf (Kaul et al. 2014). Leaf nutrient content is one indicator of nutrient availability which is critical in plant growth and development (Marschner 1986). If the production process is not balanced with the availability of nutrients, in general it will cause a decrease in production. The nitrogen and phosphorus contents of Sidimpuan snake fruit leaves were found to be higher than those of pondoh and sumedang leaves but the potassium content was lower (Islami 2014). Nitrogen status is related to leaf water content which also influences chlorophyll formation (Fig. 1). Drip irrigation increased the maximum chlorophyll content and photosynthetic nitrogen use efficiency (Wang et al. 2018). For chlorophyll production, a schedule combining drip irrigation with 300 kg Nitrogen/ha has provided the highest average chlorophyll production at an increase of 62% above non-irrigated levels (Perez-Ortola et al. 2016). Nitrogen when absorbed by plants could play an important role in the chlorophyll formation as indicated by an increase in the green leaf color (Pangaribuan et al. 2018). Fertilization increases the soil nutrient availability for the plants to absorb. The average nitrogen and potassium levels of the Sidimpuan snake fruit leaves were increased after the application of ammonium sulfate fertilizer (Adelina et al. 2018). The higher the frequency of fertilization, the more secured is the availability of soil nutrients for plant growth and development (Vargas & David 2015). The lack of plant response to fertilizer in this research may have been due to the plants’ need for a continuous supply of nutrients throughout the year. Hence, a better result may have been achieved if fertilizers were given more often. For further research, it is recommended to increase the dosage of ammonium sulfate and potassium chloride fertilizers to determine a possible increase in the the fruit set percentage and production of Sidimpuan snake fruit. CONCLUSSION Drip irrigation during the off season in July- September has improved both the fruit set percentage and the number of harvested fruit bunches of the Sidimpuan snake fruit. The best treatment, as compared to no drip irrigation, was the irrigation with 3000 mL/plant/day. As a single separate factor, the ammonium sulfate fertilizer treatment and the drip irrigation application significantly influenced the fruit set percentage and number of formed fruit bunches. No significant interaction existed between irrigation and fertilization. However, Sulfate ammonium fertilizer on the off-season production of snake fruit (Salacca sumatrana Becc.) – Adelina et al. 163 based on the average number, the combined treatments that gave the best response was obtained at the ammonium sulfate fertilizer dosage of 400 g/plant. With drip irrigation, the off-season harvest in July-September was comparable to that of the fruit set percentage and number of fruit bunches formed with fertilization, during the main harvest season in April-June. ACKNOWLEDGEMENTS This research was funded by DP2M Dikti, through a doctoral dissertation grant in the 2018 implementation year. The researchers, therefore, would like to express their deepest gratitude to the Director of DP2M Dikti for the funding support. 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